Post-tensioned construction is a construction technique in which portions of a structure, such as a bridge, are secured to each other using “tendons” that extend throughout the structure. The tendons comprise an outer duct through which steel strands extend. Once the tendons have been placed into position, the strands are tensioned to provide rigidity to the structure.
In order to prevent corrosion of the steel strands and improve mechanical performance, the ducts are filled with a grout material, which typically comprises a mixture of cement and water. When the grout is properly distributed within the duct, it creates a chemical environment that protects the steel. When the grout is not properly distributed, however, corrosion can occur. For example, if air gaps exist within the duct, the portions of the strands within those portions are exposed and may corrode. Alternatively, if the grout is not mixed properly or the mixture separates, regions that only contain water can be formed, which also can lead to corrosion.
There are various methods that can be used to determine if there is a problem with the grout within a duct. For example, there are electromagnetic methods that can be used to measure the dielectric properties of the grout mixture. Acoustic and thermal methods may be feasible as well. Unfortunately, the results that are obtained by such methods can depend upon the position of the strands within the duct. For example, one may obtain a false negative result (negative meaning the grout is not deficient) even if the grout is faulty if the testing is performed on a side of the duct at which the strands are bunched together. Alternatively, an erroneous positive indication of a deficiency (that is, a false positive) may be obtained if the testing is made on a side of the duct were the grout is in good condition, but where the strands are farther away from the sensor because of bunching on the opposite side. An indication of the position of the strands within the duct is therefore necessary for appropriate interpretation of electromagnetic, acoustic, or thermal measurement results. Although the position of the strands within the duct can be determined using x-ray imaging, such a process is complicated, time consuming, and expensive.
In view of the above discussion, it can be appreciated that it would be desirable to have an alternative way to determine the position of steel strands within a tendon used in a post-tensioned segmental structure.